1. Field of the Invention
The present invention relates generally to optical devices for mode-transforming interconnections, and more particularly, to a multi-lens mode-transforming apparatus configured to facilitate high efficiency coupling of optical signals passed between optical components and/or other waveguides having different mode fields.
While the present invention is subject to a wide range of applications, it is particularly well suited for coupling sources of elliptically-shaped optical signals, such as laser diodes and semiconductor waveguides, to optical fibers having circularly symmetric mode fields.
2. Technical Background
Coupling optical signals passed between signal sources, such as laser diodes, optical fibers, and Semiconductor Optical Amplifiers (SOAs), and other optical components, such as optical fibers, specialty fibers, SOAs and the like, with a high coupling efficiency, is an important aspect of optical communications. A conventional light-emitting module incorporated in an optical communications system generally includes a semiconductor laser, such as a laser diode, serving as a light source, an optical fiber having a light carrying core, and a lens such as a spherical lens, self-focusing lens or aspherical lens interposed between the semiconductor laser and optical fiber for converging the laser beam onto the optical fiber core. Since the light-emitting module typically requires high coupling efficiency between the semiconductor laser and the optical fiber, the module is preferably assembled with the optical axes of the semiconductor laser, lens, and optical fiber aligned with each other in order to achieve maximum coupling power. The relatively large size and high cost of early light-emitting modules, due in part to lens spacing and alignment issues, have driven advancement in the field and have resulted in a number of alternative approaches.
One such approach is the use of a graded-index (GRIN)-rod lens. Unlike other lenses, the index of refraction of a GRIN-rod lens is radially-dependent and is at a maximum at the optical axis of the rod lens. Generally speaking, the refractive index profile across a GRIN-rod lens is parabolic in shape, and thus it is the lens medium itself, rather than the air-lens interface, that performs the lensing. Accordingly, unlike conventional lenses, GRIN-rod lenses have planar input and output surfaces making refraction at these surfaces unnecessary. This characteristic enables optical elements at either end of the lens to be fixed in place with index-matching glue or epoxy. The index gradient is typically produced by an ion-exchange process that is both time-consuming and expensive. A typical GRIN-rod lens, for example, may be produced by ion-exchange of thallium or cesium-doped silica glass. A molten salt bath may be used for the ion-exchange process such that sodium and either thallium or cesium ions diffuse out of the glass, while potassium ions diffuse into the glass from a 500° C. KNO3 bath.
Since it is the refractive index profile of the lens medium resulting from this process that lenses the light, tight controls are required during the manufacturing process to ensure that a given GRIN-rod lens has the appropriate refractive index profile for a particular coupling application. Moreover, unlike GRIN-fiber lenses employed in accordance with the present invention, GRIN-rod lenses are poorly adapted for splicing to standard telecommunication fibers, and/or other optical components. Generally speaking, GRIN-rod lenses are multi-component glass structures that have significantly different coefficients of thermal expansion and softening points (the temperature at which the glass softens) than the optical waveguides to which they are coupled. GRIN-fiber lenses, on the other hand are typically made by a fiber manufacturing process and are high silica composition structures. Thus, the softening points and thermal expansion coefficients of GRIN-fiber lenses are substantially similar to the softening points and thermal expansion coefficients of most telecommunication fibers and other waveguides to which they may be attached. Accordingly, GRIN-fiber lenses are well adapted to be coupled, as for instance, by fusion splicing, to most telecommunications fibers.
Another approach has been to form a microlens on an end of an optical fiber to provide optical coupling between a semiconductor laser and an optical waveguide. In such an approach, the lens is directly and integrally formed on an end face of the optical fiber at a portion of the fiber on which light from the light source is incident. Such an optical fiber is hereafter referred to as a, “lensed optical fiber”. When manufacturing light-emitting modules using such lensed optical fibers, the number of required component parts can be reduced since there is no need for light-converging lenses apart from the fiber itself, and since the number of operations associated with axial alignment may also be reduced. Lensed optical fibers are referred to as anamorphic lensed optical fiber when the lens formed on the end of the optical fiber is capable of changing the mode field of an optical signal passed therethrough. More specifically, an anamorphic lens formed on the end of the optical fiber is generally capable of changing the elliptical mode field of an optical signal emitted from a laser diode to a substantially circularly symmetric optical signal, which can be more efficiently coupled to the core of an optical fiber having a circularly symmetric mode field.
Each of the above-described approaches have various utilities and advantages that are well known in the art. Each approach does, however, have its own set of limitations. For example, while conventional GRIN-rod lens technology provides excellent symmetrical focusing characteristics for optical signals passed therethrough, GRIN-rod lenses alone generally do not significantly alter the geometric shape of an optical signal as is often required for efficient optical signal coupling applications. In addition, since it is the material characteristics of the GRIN-rod lens itself that provides the focusing, precise manufacturing techniques are necessary in order to provide controlled variation of the refractive index profile of the GRIN-rod lens needed for a particular application.
Likewise, while anamorphic fiber lenses readily facilitate the changing of the geometric shape of the optical signal or beam passing through them, the range of available working distances for anamorphic lens applications is somewhat limited. Accordingly, if suitable working distances are not available for particular applications, coupling losses may be significant, thereby making many coupling applications impractical.
What is needed therefore, but presently unavailable in the art, is a multi-lens apparatus for optical signal coupling applications that overcomes these and other shortcomings associated with the use of anamorphic lenses or GRIN-rod lenses alone. Such a multi-lens apparatus should be capable of changing the geometric shape and other mode field characteristics of an optical signal passing through the apparatus, while at the same time providing design flexibility that will limit coupling losses, allow a broader range of acceptable working distances, minimize phasefront aberrations, and generally provide greater control and efficiency in optical signal coupling applications. Such a multi-lens apparatus should be relatively inexpensive to manufacture, be relatively easy to mass produce, and in general, have a far broader range of applications without significantly altering the material properties and characteristics of the lenses themselves. It is to the provision of such a multi-lens device that the present invention is primarily directed.